1. Field of the Invention
The present invention is directed to an improved process for the steam reforming of hydrocarbon gas feeds, and specifically to a steam reforming process which utilizes series steam superheaters for improved heat integration.
2. Description of the Prior Art
Generally, the manufacture of ammonia consists of preparing an ammonia synthesis gas from a nitrogen source, usually air, and from a hydrogen source, which is conventionally either coal, petroleum fractions, or natural gases. For example, in the preparation of ammonia synthesis gas from a light hydrocarbon feedstock, which may range from natural gas to naphtha, the hydrocarbon feedstock gas is first purified by removing gaseous contaminants, such as sulfur (which would poison the downstream catalysts) from the feedstock by the catalytic hydrogenation of the sulfur compounds to hydrogen sulfide and adsorption of the hydrogen sulfide over a zinc oxide adsorption medium. Subsequent steam reforming of the contaminant-free gas provides the major portion of the hydrogen required for ammonia synthesis from the hydrocarbons in the gas. Reforming is accomplished by a two-stage process in which a mixture of steam and the purified feed gas are first reformed over catalyst in a primary reformer, followed by treatment of the partially reformed gas in a secondary reformer to which air is introduced, in order to provide the required amount of N.sub.2 for ammonia synthesis. A reformed gas is produced in the secondary reformer having a greater amount of hydrogen and a lesser amount of hydrocarbons. The reaction processes occurring in the reforming of the feedstock gas begin with the breakdown of hydrocarbons to methane, carbon dioxide and carbon monoxide:
H.sub.2 O+C.sub.n H(2n+2).fwdarw.CH.sub.4 +CO+CO.sub.2 +H.sub.2 PA1 CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 PA1 2CH.sub.4 +7/2 O.sub.2 .fwdarw.CO.sub.2 +CO+4H.sub.2 O PA1 CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 PA1 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O PA1 CO+1/2O.sub.2 .fwdarw.CO.sub.2 PA1 N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3
and end with the reforming of these products by the desired endothermic methane reforming reaction:
and by accompanying exothermic reactions:
The carbon monoxide in the reformed gas is converted to carbon dioxide and additional hydrogen in one or more shift conversion vessels, and the carbon dioxide is removed by scrubbing. Further treatment of the raw synthesis gas by methanation may be used to remove additional carbon dioxide and carbon monoxide from the hydrogen-rich gas, resulting subsequently in an ammonia synthesis gas containing approximately three parts of hydrogen and one part of nitrogen, that is, the 3:1 stoichiometric ratio of hydrogen to nitrogen in ammonia, plus small amounts of inerts such as methane, argon and helium. The ammonia synthesis gas is then converted to ammonia by passing the gas over a catalytic surface based upon metallic iron (conventionally magnetite) which has been promoted with other metallic oxides, and allowing the ammonia to be synthesized according to the following exothermic reaction:
Conventional reforming ammonia plant designs recover all of the waste heat available from cooling the secondary reformer effluent (typically at temperatures of from 1600.degree. to 1900.degree. F.) down to a temperature suitable for high temperature shift converter operation (which typically employs an inlet temperature of from 600.degree. to 750.degree. F.) by generation of high pressure saturated steam. Since saturated steam temperatures at pressures within the realm of proven steam turbine technology are no more than about 650.degree. F., this results in a substantial temperature downgrading of the heat available in the secondary reformer effluent, which is undesirable from the standpoint of the second law of thermodynamics. Even more importantly, it represents a severe restriction on steam balance flexibility since the waste heat goes only into saturated steam generation.
In U.S. Pat. No. 3,441,393 to Pullman, a series of two saturated steam generators are employed to recover heat from the secondary reformer effluent.
Various patents to ICI have issued in which the secondary reformer effluent is first employed to generate saturated steam in a first steam generator and is then used to generate superheated steam from steam passed thereto from a steam superheater which in turn has recovered heat from the ammonia synthesis reactor effluent. Finally, additional quantities of saturated steam are generated in a second steam generator from the secondary reformer effluent, before this effluent is passed to the shift conversion.
U.S. Pat. Nos. 4,213,954 and 4,264,567 illustrate these systems. U.S. Pat. No. 4,367,206 to ICI produces methanol and ammonia in a process in which heat is recovered from the effluent of one ammonia synthesis reactor catalyst stage by superheating steam which is then further superheated by indirect heat exchange with secondary reformer effluent. However, such a process is not easily employed in combination with newer ammonia synthesis reactor designs which employ large volumes of conventional catalyst to improve energy efficiency and which optimally operate at lower temperatures, nor with newer ammonia synthesis catalysts which also optimally operate at lower temperatures. Also, the process requires the use of expensive, high pressure shells in said superheat exchanger equipment since, if energy efficient high pressure steam generation is employed, both fluids exchanging heat would have pressures exceeding 1000 psig (1000-2200 psig on the steam side and 1000-6000 psig on the reactor effluent side).
U.S. Pat. No. 3,442,613 to Braun also illustrates a system in which saturated steam is generated by heat recovery from the secondary reformer effluent. Another patent in this vein is U.S. Pat. No. 3,947,551 to Benfield.